Hi Paul,
Does the issue always happen to those specific emails? Or do you mean the content of a specific email can be displayed properly sometimes, and displayed well for other times?
Is there any special content for the problem emails?
What IE version is being used when the issue happen? IE11? IE10? IE9 or IE8, etc. Or the issue is seen on all these IE versions?
Is there any other symptom when the problem happens? For example, JS error occurs, or timeout happens, etc.
Please check following sap notes:
1145782 - Javascript error occurs randomly when viewing emails
1613256 - Session Time Out occurs on navigating away from email view
Best Regards, Corrine